What are the uses of reactive distillation

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Many products are made by means of a chemical reaction and subsequent distillation. If these two processes could be combined with one another and combined in one process step, new perspectives and great advantages would open up, for example a higher yield as well as less equipment and energy consumption.

The author: Dr. Daniel Bethge Head of Research and Development, GIG Karasek

The combination of chemical reaction and distillation, the reactive distillation, has so far been used in esterification reactions, in the production of potassium, in the production of MTBE (methyl tert-butyl ether). A combination is possible here because the operating conditions allow it. One or more products can be removed by evaporation during the reaction. As a result, the chemical equilibrium is always readjusted and shifted in the desired direction. In addition to the particularly good yield and conversion, the process is inexpensive. Instead of a reactor and a distillation device, only one device needs to be operated.

In principle, any distillation apparatus can be used if appropriate, e.g. B. a simple bubble, a rectification column or an evaporator. The residence times of the substances in the bubble are usually quite long (comparable to those in a stirred tank reactor). In the case of temperature-sensitive products or to avoid the formation of higher molecular weight components, it would be beneficial to set a shorter and, above all, defined residence time. Rectification columns with special column internals are therefore suitable for many applications. A large number of separation stages allow the products to be drawn off in the desired purity. The disadvantage of the column is the distribution of the residence time of the substances in it. The conditions are completely different in the ideal flow pipe, in which a piston flow prevails. The dwell time is the same for each particle.

Mixture of substances as a thin film

The conditions in the thin-film evaporator are the same as those in the flow tube. The mixture of substances to be evaporated is applied as a thin film to the inside of an externally heated cylinder and circulated with the help of a wiping system. The volatile components evaporate and are deposited in an external condenser. Heavy boilers stay behind. Thanks to the circulation that creates turbulence in the film, the residence time is the same for each non-evaporated particle. The typical residence time distribution curve rises very quickly after a dead time, has a pronounced maximum and drops steeply. The mean residence time decreases with increasing throughput and / or decreasing viscosity due to the higher flow rate. The dwell time in the thin-film evaporator can be controlled by selecting suitable wiping elements. As the speed increases, the hold-up, the amount of product in the apparatus, increases significantly. This increases the length of stay.

The conditions in a falling film evaporator in single-pass operation are very similar, provided the flow is turbulent. However, due to the usually lower viscosity and the high irrigation density, the times are much shorter.

Tested in the laboratory

GIG Karasek optimizes the operating conditions with the help of laboratory and pilot tests. The apparatus technology used from thin-film evaporation has proven itself over decades, because it enables the best results to be achieved. For example, in a condensation reaction in which monomers are polymerized with elimination of water, the starting materials are mixed, heated and fed evenly into the thin-film evaporator. Throughput and circulation are selected so that the residence time required for the desired conversion is achieved. With every compound reaction, a water molecule is released. Due to the applied vacuum, the water is continuously drawn off and condensed via the gas phase. The amount of distillate is directly proportional to the degree of polymerization. Depending on the operating conditions, special requirements are placed on the vacuum system.

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